Method for feeding sheets

ABSTRACT

A method for feeding a plurality of sheets from a stack of sheets into a sheet processing machine includes picking up a first sheet from the stack of sheets using a first feeding device. The sheet is conveyed to a first transfer point of the processing machine. A second sheet is picked up from the stack of sheets using a second feeding device and conveyed to a second transfer point. The first and second transfer points are spatially separated. The sheets alternately move into a sheet conveyor device of the sheet processing machine. A single transfer point can also be used.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority of German Patent Application No. 102011016105.8, filed Apr. 5, 2011, which is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to a method for feeding a plurality of sheets into a sheet processing machine. The method is particularly suitable for a digital printing machine.

BACKGROUND OF THE INVENTION

In a machine for processing sheets, individual sheets are fed one after the other. To do so, the sheets can be stored as a stack of sheets in a feeder adjacent to the sheet processing machine and are individually lifted off the stack of sheets by conveyor elements. Known conveyor elements are, for example, conveyor belts, rollers, and suction nozzles. Feeding of the sheets is accomplished at a speed that is lower than the processing speed. The lifted-off sheets are transported to a transfer point into a sheet conveyor device. Beginning at the transfer point, the sheets are continued to be transported in the sheet conveyor device and guided on a sheet path to various processing stations. Such processing stations are, for example, alignment units, printing units, and perforating units.

Recently, the achievable processing speeds in sheet processing machines have been expanded. However, it is not possible to accelerate the sheets that are stacked in the feeder in any order. On the one hand, in an acceleration too strong, the sheets can be damaged, for example due to tears or waves. This leads to sheet jams or processing errors. On the other hand, the conveyor elements for lifting and transporting the sheets from the sheet stack in the feeder cannot be accelerated at will. If these conveyor elements are strongly accelerated, this requires a high amount of energy and significant stress occurs due to bearing forces or centrifugal forces. Due to the physical limits, the total duration of the feeding process, i.e., lifting and conveying the sheets, cannot be reduced at will. The feeding process up to the transfer of the sheet to the sheet conveyor device of the sheet processing machine is also referred to as inputting.

In the past, it has been tried, for example, to avoid damaging the sheets and conveyor elements by varying the speed profiles when the sheets are being accelerated. For example, German Patent Application 10 2005 038 321 has disclosed such acceleration profiles.

There is a continuing need to permit the use of faster input or feeding speeds of the sheets, with reduced risk of damage to the sheets or to the conveyor elements.

SUMMARY OF THE INVENTION

Various embodiments include inputting a plurality of sheets from at least one stack of sheets into a sheet processing machine, the method comprising the steps of first picking up a first sheet by means of a first feeding device from the at least one stack of sheets and of conveying the sheet to a transfer point of the processing machine. Furthermore, the method comprises the picking up of a second sheet and conveying the second sheet from the at least one stack of sheets to a transfer point of the processing machine by means of a second feeding device. In particular, the inputting of the sheets at the transfer point by means of the first and second feeding devices is accomplished in such a manner that the sheets alternately move into a sheet conveyor device of the sheet processing machine. As a result of this, faster input or feeding speeds of the sheets are made possible, with reduced risk of damage to the sheets or the conveyor elements. In particular, the method is suitable for a digital printing machine.

The process of picking up and conveying the sheets can include picking up a sheet from the stack of sheets at a pick-up speed, then accelerating the sheet to an approximate speed of the sheet conveyor device in the processing machine, transferring the sheet to the sheet conveyor device of the processing machine and decelerating the feeding device to the pick-up speed. Using this method, strong forces acting on the feeding device are avoided.

In various embodiments, the alternating pick-up and transport of the sheets preferably takes place by starting the feeding operation of the first and second feeding devices with a time offset. In this embodiment, the sheets can be input at any distance and with any feeding frequency. Furthermore, the sheets can be conveyed over sheet moving paths of any length to the sheet processing machine.

In various embodiments, the sheets are guided from the first feeding device via a first sheet moving path to the processing machine, and the sheets from the second feeding device are guided via a second sheet moving path of a different length to the processing machine. This feeding of the sheets with a time offset is advantageously achieved by simultaneously picking up the sheets and by transporting them in the first and second sheet moving paths for different lengths of time. In this embodiment, the drive and the control of the feeding devices can be simplified because the sheets are picked up simultaneously.

In various embodiments, the first and second feeding devices, alternately and with a time offset, take sheets of the same stack of sheets. In this embodiment, design space can be saved.

Various embodiments permit using high input or feeding speeds of the sheets with reduced risk of damaging the sheets or the transport elements.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, as well as additional details and advantages of various embodiments, will be explained hereinafter with the use of preferred exemplary embodiments and with reference to the figures. They show in:

FIG. 1 a schematic of a sheet processing machine and associate feeders in accordance with an exemplary embodiment;

FIG. 2 a representation of a speed profile of a sheet in the course of a method for inputting sheets in accordance with various embodiments, which method can be used in the sheet processing machine in accordance with FIG. 1;

FIG. 3 a schematic of a sheet processing machine and associate feeders in accordance with another exemplary embodiment of various embodiments;

FIG. 4 a representation of a speed profile of a sheet in the course of a method for inputting sheets in accordance with various embodiments, which method can be used in the processing machine in accordance with FIG. 3; and

FIG. 5 a schematic representation of a part of a sheet processing machine and associate feeders, wherein the profile of a sheet transport path in the sheet processing machine is shown in detail.

To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. The figures are for purposes of illustration and are not necessarily to scale.

DETAILED DESCRIPTION OF THE INVENTION

It should be noted that in the description hereinafter the expressions above, below, right and left, as well as similar data, relate to the alignments or arrangements in the figures and are intended only for the description of the exemplary embodiments. However, these concepts are not to be understood in terms of a restriction.

FIG. 1 shows a sheet processing machine 1 as well as two feeders 3 and 5 adjacent to the sheet processing machine 1. A first sheet moving path 7 extends from the first feeder 3 to a transfer point 9 on the sheet processing machine 1. A second sheet moving path 8 extends from the second feeder 5 to the transfer point 9 on the sheet processing machine 1. The first and second sheet moving paths 7, 8 are defined by rollers, webs or guide elements that are not specifically shown. The transfer point 9 is the location where a sheet conveyed from a feeder 3, 5 is transferred in a not specifically shown sheet conveyor device to the sheet processing machine 1. The first feeder 3 and the first sheet moving path 7 can thus be referred to as the first feeding device 11, and the second feeder 5 and the second sheet moving path 8 represent a second feeding device 13.

Hereinafter, the operation of the sheet processing machine 1 and the first and second feeding devices 11, 13 in accordance with a first exemplary embodiment with reference to FIG. 2 is described. During operation, a first sheet is picked up from a not specifically illustrated stack of sheets in the first feeder 3. The sheet is picked up, for example, by suction belts, suction sensors or by similar known conveyor elements. The first sheet is then guided along the first sheet moving path 7 in the direction toward the transfer point 9 on the processing machine 1. Then, a second sheet is similarly picked up from a not specifically illustrated stack of sheets in the second feeder 5. The second sheet is then conveyed along the second sheet moving path 8 to the transfer point 9. Consequently, the first sheet is conveyed by the first feeding device 11 to the transfer point 9, and the second sheet is conveyed by the second feeding device 13 to the transfer point 9.

However, the first and second sheets are not simultaneously transported to the transfer point 9 but in a manner such that, alternately, a first sheet from the first feeding device and, subsequently, a second sheet from the second feeding device 13 reaches the transfer point 9. The pick-up and feeding of the first and second sheets are accomplished in accordance with a speed profile as shown by FIG. 2.

In a first step, the sheet is taken off the stack of sheets with the use of a conveyor element at a first pick-up speed, i.e., section 16. Subsequently, the sheet is accelerated to the approximate speed of the sheet conveyor device in the processing machine 1, i.e. section 17. As mentioned hereinabove, the acceleration, i. e. the slope of section 17, cannot be increased at will. The acceleration to the approximate speed, in this case, means an acceleration to approximately 90-100% of the speed of the sheet conveyor device in the processing machine. During the transfer at the transfer point 9, the speed of the sheet conveyor device, as a rule, should be slightly greater than the speed of the delivered sheet in order to avoid that the sheet is being decelerated. If the sheet is being decelerated, the probability increases that the sheet will become wavy and cause a sheet jam. Then, the sheet is transferred to the sheet conveyor device of the sheet processing machine, i.e., step 18. Finally, the conveyor element is decelerated to the pick-up speed, i.e., step 19. The total duration of the steps 16, 17, 18, and 19 is t, as shown by FIG. 2.

In the embodiment of FIGS. 1 and 2, the speed profile of the first feeding device 11 is offset by a period Δt relative to the speed profile of the second feeding device 13. As is shown in FIG. 2, first a sheet is picked up by the second feeding device 13 and transported, and, after the time Δt has elapsed, a second sheet is picked up and transported by the first feeding device 11. As a result of this time offset of the speed profiles of the feeding device 11 and of the second feeding device 13, the sheets alternately move into the sheet processing machine 1 at the transfer point 9.

Depending on the desired distances between the alternately entering sheets, it is possible to vary the period Δt. The period Δt can be t/2, for example. If Δt equals t/2, the sheets alternately move at the same distances from the first feeding device 11 and the second feeding device 13 into the sheet processing machine 1. If Δt equals to t/3, the sheets move at different distances into the sheet processing machine 1.

FIG. 3 is a view similar to that of FIG. 1, wherein the same or similar elements have the same reference signs, and only their differences are described. In the embodiment of FIG. 3, the first and second sheet moving paths 7, 8 have different lengths. The difference of the lengths is indicated by Δs in FIG. 3. During operation, feeding or inputting of the sheets at the transfer point 9 occurs likewise alternating from the first feeding device 11 and the second feeding device 13.

FIG. 4 shows a speed profile of a sheet that is transported by the first feeding device 11, and a second sheet that is transported by the second feeding device 13. The speed profile has the same sections 16, 17, 18, and 19, as have been described above with reference to FIGS. 1 and 2. Referring to FIG. 4, it can be seen that the first and the second sheets are picked up at the same time. The speed profiles of the feeding devices 11 and 13 have the same length, as indicated by the dashed lines in FIG. 4, and are time-synchronized. The difference of the length Δs can correspond to the length of the first sheet moving path 7, for example, so that the second sheet moving path 8 is twice as long as the first sheet moving path 7.

The feeding of the sheets with a time offset at the transfer point 9 is achieved by different transport times in the first and second sheet moving paths 7, 8, the paths having different lengths. The transport in the first sheet moving path 7 can last 1/10 sec., for example, and the transport in the second sheet moving path 8 can last 2/10 sec. The time profile of the method for inputting the sheets would then be as follows: A first and a second sheet are picked up in the first and second feeders 3, 5 at the same time. The first sheet of the first feeding device 11 reaches the transfer point 9 after 1/10 sec.; and the second sheet of the second feeding device 13 reaches the transfer point 9 after 2/10 sec. Δt the moment when the second sheet has reached the transfer point 9, i.e., after 2/10 sec., a third and a fourth sheet are picked up at the same time in the first and second feeders 3, 5. The third sheet of the first feeding device 11 reaches the transfer point 9 after a total of 3/10 sec.; and the fourth sheet of the second feeding device 13 reaches the transfer point 9 after 4/10 sec. Thus, a sheet from one of the two feeding devices 11, 13 reaches the transfer point 9 every 1/10 sec. Consequently, the sheet processing machine is able to process a sheet every 1/10 sec., even though a sheet is picked up only every 2/10 sec.

There is yet another embodiment that is taken into consideration and that is not specifically shown in the figures. In this embodiment, two feeding devices are provided, each comprising its own conveyor elements but sharing a common sheet supply or stack of sheets. The first and second conveyor elements of the first and second feeding devices are, for example, suction fingers or vacuum suction belts that can be moved independently of each other to the shared stack of sheets. Furthermore, the first and the second feeding devices comprise a shared sheet moving path that leads from the conveyor elements to the transfer point on the sheet processing machine.

During operation, the first and second conveyor elements, alternately and with a time offset, access the shared stack of sheets. This means that, first, the conveyor elements of the first feeding device pick up a sheet and transport the sheet to the transfer point on the sheet processing machine in accordance with the speed profile described above. After a period of Δt has elapsed, the conveyor elements of the second feeding device then access the same stack of sheets and pick up a second sheet. After the period Δt, the conveyor elements of the first feeding device have already transported the first sheet over a certain distance Δs along the shared sheet moving path away from the stack of sheets. The conveyor elements of the second feeding device then transport the second sheet also along the shared sheet moving path to the processing machine. The leading edges of the first and second sheets are at the distance Δs from each other and move with a time offset Δt to the transfer point on the sheet processing machine. While the conveyor elements of the second feeding device accelerate the second sheet, the conveyor elements of the first feeding device have already moved through the speed profile (see FIGS. 2 and 4) and are again ready to lift a subsequent (third) sheet off the shared stack of sheets.

Hereinafter, reference is made to FIG. 5 in order to describe another option for inputting sheets into a sheet processing machine 1. Again, the same elements have the same reference signs, and it is mainly the differences from the aforementioned embodiments that will be discussed.

The first and the second feeders 3, 5 are connected with a sheet conveyor device 22 of the processing machine 1 via first and second sheet moving path 7, 8. The first and second sheet moving paths 7, 8, however, do not end in a single transfer point 9 into the processing machine 1 but in two spatially separated transfer points 9′ and 9″. The transfer point 9′ is the point at which a sheet from the first feeding device 11 is transferred to the sheet conveyor device 22 of the sheet processing machine 1. The transfer point 9″ is the point at which a sheet from the second feeding device 13 is transferred to the sheet conveyor device 22 of the sheet processing machine 1. The sheet conveyor device 22 of the processing machine 1 includes, for example, deflecting rollers 24 and various sheet conveying sections 25. Additional elements of the sheet processing machine 1 are an alignment unit 26 and a printing unit 27.

The operation of the embodiment shown by FIG. 5 can take place in accordance with the principle described with reference to FIGS. 1 and 2 or in accordance with the principle described with reference to FIGS. 3 and 4. Each of the speed profiles of the first and second feeding devices 11, 13 has the same sections 16, 17, 18 and 19 that have been described above with reference to FIGS. 1 and 2.

In one embodiment, a time difference Δt between the starting points of the speed profiles for transporting the sheets is selected in such a manner that a desired distance Δs results between the sheets when the sheets finally move into the alignment unit 26 or the printing unit 27. The time difference Δt is a function of the distance between the transfer points 9′, and of the length of the first and second sheet moving paths 7, 8.

In another embodiment, the first and second sheet moving paths 7, 8 have the same length. The transfer points 9′ and 9″ are separated by Δs from each other. In this case, the first and second sheets are simultaneously taken off the stacks of sheets in the first and second feeders 3 and 5 and transported, during the same period, in the equally long first and second sheet moving paths 7, 8. Thus, the first and second sheets simultaneously reach the transfer points 9′ and 9″ and are separated by Δs from each other in the sheet conveyor device 22.

The description hereinabove assumes two feeding devices. However, it will be obvious to the person skilled in the art that three or more (e.g., n) feeding devices can be provided in order to achieve additional increases of the processing speed and the feeding frequency. In such a case, the time Δt would equal t/3 or t/n in order to achieve the same distances between fed sheets.

The above-described arrangements and methods can be advantageously used in printing machines, in particular in digital printing machines for printing sheets. However, it should be noted that the principles of the present invention can also be applied to other machines for sheet processing, for example, machines for cutting, stapling, perforating and gluing sheets, or to postage meter machines.

The invention is inclusive of combinations of the embodiments described herein. References to “a particular embodiment” and the like refer to features that are present in at least one embodiment of the invention. Separate references to “an embodiment” or “particular embodiments” or the like do not necessarily refer to the same embodiment or embodiments; however, such embodiments are not mutually exclusive, unless so indicated or as are readily apparent to one of skill in the art. The use of singular or plural in referring to the “method” or “methods” and the like is not limiting. The word “or” is used in this disclosure in a non-exclusive sense, unless otherwise explicitly noted.

The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations, combinations, and modifications can be effected by a person of ordinary skill in the art within the spirit and scope of the invention.

PARTS LIST

-   1 sheet processing machine -   3 feeder -   5 feeder -   7 first sheet moving path -   8 second sheet moving path -   9 transfer point -   9′ transfer point -   9″ transfer point -   11 first feeding device -   13 second feeding device -   16 section -   17 section -   18 transfer step -   19 deceleration step -   22 sheet conveyor device -   24 deflecting rollers -   25 sheet conveying section -   26 alignment unit -   27 printing unit -   t duration -   v sheet velocity -   Δs difference of lengths 

1. A method for feeding a plurality of sheets from a stack of sheets into a sheet processing machine, in particular a digital printing machine, the method comprising: picking up a first sheet from the stack of sheets using a first feeding device and conveying the sheet to a first transfer point of the processing machine, picking up of a second sheet from the stack of sheets using a second feeding device and conveying the second sheet to a second transfer point of the processing machine, the first and second transfer points being spatially separated, so that the sheets alternately move into a sheet conveyor device of the sheet processing machine.
 2. The method according to claim 1, wherein the steps of picking up include: picking up the sheet at a pick-up speed; accelerating the sheet to an approximate speed of the sheet conveyor device in the processing machine; transferring the sheet to the sheet conveyor device of the processing machine; and decelerating the feeding device to the pick-up speed.
 3. The method according to claim 1, wherein the alternating pick-up and transport of the sheets is accomplished by starting the feeding process of the first and the second feeding devices with a time offset.
 4. The method according to claim 3, wherein the sheets of the first feeding device are guided via a first sheet moving path to the processing machine, and the sheets of the second feeding device are guided via a second sheet moving path of a different length to the processing machine; wherein the feeding of the sheets with a time offset is accomplished by the simultaneously picking up the sheets and by transporting them with a time offset in the first and second sheet moving paths.
 5. The method according to claim 1, wherein the first and second feeding devices alternately and with a time offset take the sheets off the same stack of sheets.
 6. A method for feeding a plurality of sheets from a stack of sheets into a sheet processing machine, in particular a digital printing machine, the method comprising: picking up of a first sheet using a first feeding device from the stack of sheets and conveying the sheet to a transfer point of the processing machine, picking up of a second sheet using a second feeding device from the at least one stack of sheets and conveying the second sheet to the transfer point of the processing machine, so that the sheets alternately move into a sheet conveyor device of the sheet processing machine.
 7. The method according to claim 6, wherein the alternating pick-up and transport of the sheets is accomplished by starting the feeding process of the first and the second feeding devices with a time offset.
 8. The method according to claim 7, wherein the sheets of the first feeding device are guided via a first sheet moving path to the processing machine, and the sheets of the second feeding device are guided via a second sheet moving path of a different length to the processing machine; wherein the feeding of the sheets with a time offset is accomplished by the simultaneously picking up the sheets and by transporting them with a time offset in the first and second sheet moving paths. 